1. Field of the Invention
The present invention relates to stretched blow molding articles having excellent heat stability and mechanical strength which are prepared by using a special aliphatic polyester composition with rapid biodegradability and sufficiently high molecular weight and specific melt properties for practical use.
2. Discussion of the Background
Recently, with the advancement of the packaging industry, plastic package materials have been developed, and the resulting waste of a large amount of plastics used can pollute rivers, oceans, soil and the like, and has become a serious community problem. To prevent such pollution the development of biodegradable plastics has been desired earnestly; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of modified ethylene-vinyl acetate copolymer and starch of a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a thermal decomposition temperature close to its melting point and raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer blends have defects in molding properties and poor water resistance, and is greatly limited in its range of applications.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because enough high molecular weight material to obtain a practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposals have been made to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point under 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of cyclic dimers of glycolida and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as packaging materials because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
It is no exaggeration to say that high molecular weight polyesters (referring to polyesters having number-average molecular weights of at least 10,000) generally used for packaging materials, in particular, stretched blow molding articles (bottles) having a water proofness and high strength, are limited to a PET-Bottle which is obtained from polyethylene terephthalate, a polycondensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Polyethylene terephthalate is aromatic, and therefore it has no biodegradability.
U.S. Pat. Nos. 5,306,787, 5,310,782 and 5,436,056 disclose a method for producing a biodegradable high molecular weight aliphatic polyester having a melting point of at least 70.degree. C. comprising the step of polycondensation of glycols and aliphatic dicarboxylic acids. Further, U.S. Pat. No. 5,324,556 discloses blow-molded articles consisting of an aliphatic polyester. Furthermore, biaxially stretched blow-molding articles consisting of an aliphatic polyester are disclosed by some of the present inventors (Japanese Patent Laid-Open No. 5-16913 corresponding to Application No. 5-117408).
However, nothing in the prior art discloses or suggests a special aliphatic polyester composition of the present invention.
An object of the present invention is to provide stretched blow molding articles prepared by using a special aliphatic polyester composition which has a sufficiently high molecular weight for practical use and excellent mechanical properties represented by heat stability, tensile strength and gas barrier properties, and which can be rapidly decomposed by microorganisms and the like i.e., the articles are biodegradable as one means of waste disposal so that they may be easily disposed in landfill or in compost after the use thereof.